Causes and Control of Tuber Rots of White Yam (Dioscorea Rotumdata Poir Varieties Araba, Asana and Puna)

Abstract

Studies were undertaken to identify the pathogens associated with rotting of white yam (.Dioscorea rotundata var. Araba, Asana and Puna) in Ghana, to identify exportable white yam varieties with superior shelf life and to evaluate the effectiveness of lime and neem wood-ash in controlling rots in wounded yam tubers. A field survey was conducted among yam retailers and exporters in Accra. The results revealed that: a major constraint to yam production and marketing in Ghana is the high percentage of tuber rot of 10-30% for retailers and 10-40% for exporters; Puna is the most preferred white yam variety for export and local consumption; Some handling practices predispose tubers to rot diseases; traditional rot control measures is not widely practiced; yams shipped abroad are kept in poorly ventilated containers. Identification of the three yam varieties based on tuber characteristics, showed that the furrow markings on the corky periderm (skin) of the three white yam varieties are consistent with variety and therefore a reliable index for their identification. Isolations made from tubers of the three white yam varieties obtained from three yam markets in Accra and a yam barn at the Crop Science Department, University of Ghana Legon, and subsequent pathogenicity studies with the isolates revealed that storage rots were caused by Aspergillus flavus, Aspergillus niger, Aspergillus oryzae, Botryodiplodia theobromae, Erwinia carotovora, Fusarium culmorum, Fusarium oxysporum, Fusarium sp Penicillium brevi-compactum, Penicillium sp., Rhizopus stolonifer and Scutellonema bradys. This is the first record of the ability of A. oryzae and F. culmorum to cause rot in yam. Symptoms aiding the identification of A. niger, E. carotovora, P. brevi-compatum, R. stolonifer and S. bradys are described. Puna had the highest mean frequency of infection by pathogens (41.9% and 41.3%), followed by Asana (34.4% and 30.7%) and then Araba (23.7% and 28.0%) in the market and barn respectively. After ten months of storage Araba had the lowest weight loss of 39.31%, and a lower rotting percentage of 27.5% (P = 0.05). Even though at the 16th week all the yam tubers had sprouted, the rate of sprouting of Araba was significantly lower (P = 0.05). Lime was significantly more efficacious in suppressing decay in wounded tubers of the three yam varieties than neem wood-ash (P= 0.05). Lime also inhibited growth of all the fungal isolates in vitro.